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Commitments and Contingencies (Details) |  |  | 1 Months Ended | 3 Months Ended | 6 Months Ended | 12 Months Ended
--- | --- | --- | --- | --- | --- | ---
Apr. 01, 2021 | Aug. 18, 2017 USD ($) | Sep. 30, 2017 USD ($) | Jun. 30, 2021 USD ($) Labor_Union MW | Jun. 30, 2020 USD ($) | Jun. 30, 2021 USD ($) Labor_Union MW | Dec. 31, 2013 USD ($)
Labor Commitments [Abstract] |  |  |  |  |  |  |
Percentage of total work force in union |  |  |  | 67.00% |  | 67.00% |
Percentage of Grundartangi work force represented by the labor unions (in hundredths) |  |  |  | 88.00% |  | 88.00% |
Number of labor unions Grundartangi subsidiary entered into a new labor agreement with | Labor_Union |  |  |  | 5 |  | 5 |
Percentage of U.S. based work force represented by a union (in hundredths) |  |  |  | 58.00% |  | 58.00% |
Ravenswood Retiree Medical Benefits Changes [Member] |  |  |  |  |  |  |
Ravenswood litigation [Abstract] |  |  |  |  |  |  |
Gain (loss) related to litigation settlement |  |  | $ 5,500,000 |  |  |  |
Loss contingency accrual |  |  | 12,500,000 |  |  |  |
PBGC Agreement [Member] |  |  |  |  |  |  |
PBGC Settlement [Abstract] |  |  |  |  |  |  |
Required pension contributions above minimum |  |  |  |  |  |  | $ 17,400,000
Payment for pension benefits |  |  |  | $ 0 | $ 0 |  |
Pension contributions above minimum remaining |  |  |  |  |  | $ 9,600,000 |
Hawesville [Member] |  |  |  |  |  |  |
Power Contingencies [Abstract] |  |  |  |  |  |  |
Power supply agreement, termination notice period |  |  |  |  |  | 1 year |
Sebree [Member] |  |  |  |  |  |  |
Power Contingencies [Abstract] |  |  |  |  |  |  |
Power supply agreement, termination notice period |  |  |  |  |  | 1 year |
Santee Cooper [Member] |  |  |  |  |  |  |
Power Contingencies [Abstract] |  |  |  |  |  |  |
Power agreement, power supply, percentage | 100.00% |  |  |  |  |  |
Power supply agreement, increase production, at full capacity, percentage | 75.00% |  |  |  |  |  |
Grundartangi - HS, Landsvirkjun and OR [Member] |  |  |  |  |  |  |
Power Contingencies [Abstract] |  |  |  |  |  |  |
Power currently available under the power purchase agreement, available (in megawatts) | MW |  |  |  | 545 |  | 545 |
Grundartangi - Landsvirkjun [Member] |  |  |  |  |  |  |
Power Contingencies [Abstract] |  |  |  |  |  |  |
Power currently available under the power purchase agreement, available (in megawatts) | MW |  |  |  | 25 |  | 25 |
Power currently available under the power purchase agreement, extension (in megawatts) | MW |  |  |  | 161 |  | 161 |
Power currently available under the power purchase agreement, requested (in megawatts) | MW |  |  |  | 182 |  | 182 |
Netherlands [Member] |  |  |  |  |  |  |
Labor Commitments [Abstract] |  |  |  |  |  |  |
Percentage of Vlissingen work force represented by the labor union (in hundredths) |  |  |  | 100.00% |  | 100.00% |
Ravenswood Retiree Medical Benefits Changes [Member] |  |  |  |  |  |  |
Ravenswood litigation [Abstract] |  |  |  |  |  |  |
Litigation settlement amount |  | $ 23,000,000.0 |  |  |  |  |
Ravenswood litigation settlement installment period |  | 10 years |  |  |  | 9 years |
Litigation payment to trust |  |  | $ 5,000,000.0 |  |  |  |
Litigation settlement, amount awarded to other party, annual payments |  |  |  |  |  | $ 2,000,000.0 |
Hydrocarbon Recovery Plan [Member] | St. Croix [Member] |  |  |  |  |  |  |
Ravenswood litigation [Abstract] |  |  |  |  |  |  |
Litigation settlement amount |  |  |  | $ 600,000 |  |  |
Other current liabilities [Member] | Ravenswood Retiree Medical Benefits Changes [Member] |  |  |  |  |  |  |
Ravenswood litigation [Abstract] |  |  |  |  |  |  |
Litigation settlement, amount awarded to other party, annual payments |  |  |  |  |  | 2,000,000.0 |
Other liabilities [Member] | Ravenswood Retiree Medical Benefits Changes [Member] |  |  |  |  |  |  |
Ravenswood litigation [Abstract] |  |  |  |  |  |  |
Litigation settlement, amount awarded to other party, annual payments |  |  |  |  |  | $ 7,800,000 |

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